In the field of digital printing, a digital printer receives digital data from a computer and places colorant on a receiver to reproduce the image. A digital printer can use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers.
Modern inkjet printers are capable of delivering excellent image quality, but suffer from poor durability with respect to environmental factors such as atmospheric gases and staining fluids. For example, naturally occurring ozone is known to cause fading in inkjet prints, which are exposed to the atmosphere. The degree of fading can become unacceptable in a relatively short time period, often only a few weeks of exposure to the air. Exposure to moisture and/or staining agents can be another source for unacceptable image quality artifacts in an inkjet print. Many inkjet prints will “run” or “bleed” (where the ink begins to run off the page) when exposed to water. When subjected to other fluids such as coffee or mustard, unacceptable stains can form on the surface of the inkjet print, often in the white portions of the page where ink has not been printed. Additionally, there are optical effects that can occur with inkjet prints, which result in a perceived image quality loss. In particular, the gloss difference at the boundary between the inked and non-inked areas of the image can be disturbing to a human observer. Yet another environmental factor that can cause image artifacts in an inkjet print is handling or abrasion. Rubbing an inkjet print with a finger can cause the ink to smear from a printed area into a non-printed area, resulting in poor image quality.
The above described image artifacts can occur in inkjet prints because the surface of an inkjet print is not “sealed” or protected from the environment. Several methods to address these undesirable image artifacts are known in the art. One technique known in the art is to laminate the print, but this is typically too time-consuming and costly. Another technique is to apply an additional, substantially clear ink that has protective properties to the image during or shortly after the printing process. For example, U.S. Pat. No. 6,412,935 to Doumaux discloses an inkjet printer in which a “fixer” ink is printed using a separate printhead, which is vertically offset from the colored ink printheads. This technique involves an extra print pass where the paper is not advanced, and the fixer fluid is printed over the image. Similar techniques are described in U.S. Pat. No. 6,503,978. U.S. Pat. No. 6,443,568 to Askeland, et al., describes a method of underprinting and overprinting a clear fixer fluid, and applying heat to provide for improved water fastness.
The above mentioned references teach the use of a protective fluid for improving print durability, but do not teach methods of controlling the laydown of the protective fluid in response to the amount of colored ink that will be printed. For example, the use of pigmented inks is known to provide for some increase in durability properties when compared with dye inks. The application of a full layer of protective fluid on top of an area printed with pigmented inks is likely unnecessary to achieve the desired durability, and is wasteful of ink. Also, indiscriminate application of protective fluid leads to a dramatic increase in the total amount of fluid deposited on the page, which is known to cause other negative image quality artifacts. See for example U.S. Pat. No. 6,435,657.
Additionally, when applying a protective ink to provide for improved durability, the best protection is achieved when the surface of the receiver is completely sealed from environmental factors. If the protective ink amount is computed before the image data is halftoned (as described in commonly assigned U.S. patent application Ser. No. 10/785,818 filed Feb. 24, 2004 by Douglas W. Couwenhoven, et al., entitled “Inkjet Printing Using Protective Ink”, the disclosure of which is herein incorporated by reference), then complete coverage of the receiver can not be guaranteed, since the halftone process will result in patterns of dots of protective ink that do not necessarily fill in all of the “white holes” left by unprinted pixels.
Thus, there is a need for a method of computing a protective ink amount to be applied to an image to provide for improved durability, while minimizing the total amount of fluid deposited on the page, and ensuring complete coverage of the receiver with either protective or colored ink for maximum environmental durability.